Ring lamps of this type are employed, for example, in camera objectives used for macrophotography, as operating theater illumination in medicine or else for transmitted-light or reflected-light illumination in optical microscopy. In this context, such a ring lamp is arranged concentrically to the optical axis of the image-forming optical system. Therefore, in contrast to spot illumination, the object to be imaged is illuminated without any shadows. Shadowless, uniform and intense illumination is also needed in the field of particle detection in currents, but only within a delimited volume, as a result of which only the particles that enter the illuminated area with the current are observed.
There are various illumination devices in the field of particle detection. East German patent application DD 232 552 A1 describes a device to count and classify dispersed particles in liquids having volume to be measured that is spatially delimited by a measuring cell, a laser beam serving to illuminate the volume to be measured, whereby the beam focus lies in the center of the volume to be measured, so that the detected volume to be measured is reduced to a single point. The volume to be measured is restricted by the configuration of the measuring cell in such a way that every time only one particle is situated in the light focus and its scatter in the laser light is then measured. East German patent application DD 221 861 A1 describes an illumination device for generating a two-dimensional light strip for pattern recognition and identification of workpieces in an industrial setting. For this purpose, a linear light source is used whose beams are directed through a blade diaphragm and bundled by a cylindrical lens onto the object that is to be recognized. Contrast adjustment in the imaging system then creates and analyzes a sharp black-and-white image of the strip that is being illuminated. A reflector can be installed behind the light source for purposes of obtaining a higher light yield. Moreover, German utility model DE 298 13 109 U1 describes an illumination device for generating a long, narrow light band having two-dimensional characteristics. Here, the light from several lamps in a narrow housing with a first individual lens for each lamp and with a second, shared lens forms a narrow beam focused onto a line at a selectable distance. The light yield is approximately a function of the emitting angle and is thus very small. German patent specification DE 197 36 172 B4 describes a device for analyzing particles dispersed in a flowing liquid that works with diaphragms whose edges are curved like a hyperbola and thus, under illumination, these edges define a three-dimensional volume to be measured that has a known depth of field shaped like a truncated cone with curved edges. In the associated method, particles having a defined travel time in the volume to be measured are evaluated. The illumination device is arranged parallel to the detector, the detection signal is deflected out of the volume to be measured by a prism.
Prior-art video plankton recorders (VPR) utilize punctiform or linear light sources and individual spherical or cylinder lenses for the collimation. In large-scale applications, strong halogen systems are also used. These devices, however, entail the problem that no sharp optical delimitation of the volume to be measured can be created and the depth of field in the volume to be measured is adjusted employing complex software, which accounts for a high degree of imprecision of the estimated volume when used in the small-scale realm of plankton observation.
The ring lamp for shadowless reprophotography described in German utility model DE 299 21 150 U1 is a camera lamp. In this publication, which includes technical background for the present invention, a commercially available ring fluorescent lamp is employed as the illumination means. It emits its light uniformly to all sides, in other words, also towards the inside in the direction of the axis of the light source. Here, a device for directing the light emission is present in the form of a device that primarily changes the incidence of light onto the object to be illuminated in that the illumination means is raised or lowered. Furthermore, German patent application DE 102 11 768 A1 describes a ring lamp that is viewed in the present invention as being the closest state of the art. This publication discloses a light source consisting of a ring of light diodes for white light and configured as a flat hollow cylinder, the emitting direction of said light source being oriented inwards onto the axis of the hollow cylinder. Owing to a split construction, the diameter of the first ring that carries the light diodes can be changed relative to a stationary second ring. As a result of the fact that the lower end of the flexible, circularly curved mounting plate that carries the diodes is arranged in the stationary second ring and its upper end is arranged in the variable first ring, any change in the diameter of the first ring changes the angle of the light emission relative to the axis of the ring lamp. This device serves to optimize a shadowless illumination of several stationary objects that are to be imaged. The described device for directing the light emission, however, cannot illuminate a strictly delimited volume. Since the light is only delimited by the edges—whose dimensions are not defined—of the two rings that can be moved with respect to each other, it is freely emitted and thus spans a double conically shaped volume that is unsharply delimited towards the top and bottom and that has scatter areas that extend considerably beyond this.